-Declassified missions will add difficulty to each section, by adding a timer, different enemy variants, special weapon requirements or gameplay effects (like smoke).- After finding the declassified mission, make sure you fulfill the requirements so that it counts towards this achievement.- To check declassified mission progress, from the main menu select "Stats & Awards". On the first tab, select "Campaign". Use RB to scroll to Pages 41-46 where you can see what difficulty you've completed each declassified mission on.

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